Introduce multi-purpose data/len members in struct cmark_node. This
is mainly used to store literal text for inlines, code and HTML blocks.

Move the content strbuf for blocks from cmark_node to cmark_parser.
When finalizing nodes that allow inlines (paragraphs and headings),
detach the strbuf and store the block content in the node's data/len
members. Free the block content after processing inlines.

Reduces size of struct cmark_node by 8 bytes.

Allows to reduce size of struct cmark_node later.

Reduces size of struct cmark_node by 8 bytes.

Use zero-terminated C strings instead of cmark_chunks without storing
the length. This introduces a few additional strlen computations,
but overhead should be low.

Allows to reduce size of struct cmark_node later.

Use zero-terminated C strings instead of cmark_chunks without storing
the length. The length of code literals will be readded in a later
commit. strlen overhead for code info should be negligible.

Reduces size of struct cmark_node by 8 bytes.

Removes CMARK_OPT_SAFE from options.

Adds CMARK_OPT_UNSAFE, with the opposite meaning.
The new default behavior is to suppress raw HTML and
potentially dangerous links.  The CMARK_OPT_UNSAFE
option has to be set explicitly to prevent this.

--------------------------------------------------------
NOTE: This change will require modifications in
bindings for cmark and in most libraries and programs
that use cmark.
--------------------------------------------------------

Closes #239, #273.

Borrows heavily from @kivikakk's patch in github/cmark-gfm#123.

They're not supported by MSVC.